Summary:
The Scully Deposit mineralization style is a deposit typical of the Superior-Lake type of Iron Formations.
The Scully Mine lies within the Labrador Trough in Western Labrador. The Sokoman Formation is an iron formation that consists of three (3) iron bearing formations, named the Upper, Middle and Lower Iron Formations. The Sokoman Formation is more than 300 m thick near the Scully Mine and has been subjected to two (2) episodes of folding and metamorphism during the Hudsonian and Grenville orogenies, resulting in a complex structural pattern in the Wabush Area. The younger Menihek and Shabagamo Formations and the older Denault, Attikamagen, and Wishart Formations all outcrop in the vicinity of the mine site. The mineral deposit that defines the Scully Mine consists of folded and faulted stratigraphic beds of iron-bearing units within the regional Sokoman Iron Formation.
Mineralization
The ore minerals are hematite (specularite), magnetite, and martite. The waste minerals are quartz and hydrated iron oxides such as limonite and goethite. Manganese oxides also occur in bands or are disseminated throughout the iron-bearing units.
All units are overlain unconformably by glacial till (overburden) and underlying the Lower Member is a silicate-rich iron formation known as the Basal Silicates (considered the footwall waste unit). Unfolded, the average thickness of the formation is about 800 feet. Each member is split up into separate numeric sub-members, which reflect the changes in ore characteristics (mineralogy, hardness, manganese abundance).
Basal Silicates
Near surface, the basal silicates comprise narrow bands of quartz, limonite and goethite, with pyrolusite in bands running parallel to the bedding. Characteristically, these have high iron assays with very low weight recovery. Fresh samples from drill holes show a layered quartz-amphibole assemblage. Green biotite and chlorite are also common, and garnets become more abundant near the base of the member.
Lower Member Ore
This is generally a hard, dense, blue quartz-hematite rock, with little or no limonite (except where fibrous silicates along the bedding planes of the ore are altered). The ore is distinctly banded with segregation of hematite, magnetite and quartz. Magnetite is common, increasing toward the base of the unit. Vugs are rare and abundant manganese oxides occur in bands and disseminated throughout the ore. Near the base, the layers of hematite and magnetite give way to an amphibole assemblage, which near the surface has altered to limonite. The unit is characterized by intermediate iron and iron unit recovery values. The Lower Member contains an internal thick interbed of limonite-rich ore which presents low weight recoveries .
Middle Quartzite
This is a narrow but persistent band of quartzite which runs the entire length of the orebody. Iron minerals, including limonite and goethite, and more rarely hematite, and pyrolusite fill lenses, joints and cracks. Because of its distinct lithology, this horizon has been widely used as a marker horizon in the structural interpretation of the orebody.
Middle Member
This is invariably a coarse-grained, blue specularite-quartz rock. Hematite often makes up more than 50 percent by weight of the rock. The ore is usually soft and friable, breaking down to constituent grains on drying. This is normally explained by intense weathering and removal of silica. Elsewhere, similar leaching of the ores at Knob Lake (O’Leary, 1972) is demonstrated in part by the presence of depressions and crevices filled with lacustrine clays and argillite and containing fossil plants of Cretaceous age. Evidence of leaching of this nature on a similar scale is not found at Wabush Mine.
The magnetite content of the member is low but increases rapidly toward the base. Manganese is present as earthy pyrolusite in small vugs which vary in size from half a cm to 5 cm diameter. It also occurs disseminated through the ore, as distinct bands and included in the hematite and magnetite grains, both as separate mineral phases and in solution in the hematite and magnetite crystals. The ore is noticeably banded and has a sub micaceous high lustre. Hematite and quartz are often completely segregated into bands. Massive, hard, mixed quartz-hematite bands, with a pink hue, are also common. Hematite in this association is non-micaceous and has a dull lustre. Hematite and quartz are cemented by a fine-grained magnetite matrix. The nature of the contact between Middle Member ore and the Middle Quartzite varies, but there is often a layer of limonite-rich, oxidized ore known as sub-unit 34 which is sometimes considered waste.
Upper Member
This lies immediately beneath the hanging-wall carbonaceous quartzite and is a quartz-goethite-limonite rock with varying amounts of magnetite and specularite. Quartz grains are stained with limonite, giving this member a dirty brown appearance, and bands of colloidal goethite up to 15 cm wide are relatively common. The Upper Member varies from a hard-fine-grained through coarse grained and friable to a soft earthy variety. It is thought that the limonite coatings of quartz grains have been produced by weathering near surface. The member is characterized by low iron and a poor weight recovery, making it the least attractive of the three (3) ore members.